Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R3/00—Circuits for transducers, loudspeakers or microphones
  • H04R3/04—Circuits for transducers, loudspeakers or microphones for correcting frequency response

Definitions

• the invention relates to the field of signal processing, especially processing of audio signals. More specifically the invention provides a method and a device for processing an audio signal with the purpose of extending low frequency output when reproduced by a loudspeaker.
• Some different non-linear signal processing methods have been proposed to pre- process an audio signal in order to enhance low frequency output from such small loudspeakers. Often such pre-processing is referred to as “bass enhancement” or “low frequency bandwidth extension”. Audio bandwidth extension is known in the field of audio and described thoroughly e.g. in the book “Audio Bandwidth Extension” by Erik Larsen and Ronald M. Aarts, John Wiley & Sons Ltd. 2004, ISBN 0-470-85864-8. As described in this book, low frequency bandwidth extension can be obtained by utilizing psychoacoustic properties of the human auditory system, i.e. to provide the listener with the perception of a larger amount of low frequency content than is physically present.
• US 5,359,665 by Aphex describes another bass enhancement system including a bass compressor having a variable gain amplifier controlling gain controlled by a signal level detector sensing the level of the bass components. In this way high bass amplification is provided at low bass levels, while less bass amplification is provided at higher bass levels.
• compressors are used for reducing the dynamics of a signal. Thus the difference between the quiet parts and the loud parts is lessened, and thus the overall signal can be boosted. In order to optimise the compressor so-called attack and release times have to be set.
• the invention provides a method of enhancing the low frequency content of an input signal, including a) performing an overdriving of a low frequency signal part of the input signal to form a boosted low frequency signal, wherein the overdriving includes amplifying the low frequency signal part by a first gain to form an amplified low frequency signal, and hard-clipping the amplified low frequency signal to form the boosted low frequency signal, b) performing a first low-pass filtering of the boosted low frequency signal to form a processed low frequency signal, wherein a first cut-off frequency of the first low- pass filtering is selected so as to reduce distortion components introduced by the overdriving, and c) combining the processed low frequency signal with at least part of the input signal to form an output signal.
• hard-clipping By 'hard-clipping' is understood applying a pure gain and a saturation of the signal at a predefined maximum level. By employing hard-clipping, low level signals will be increased by a linear gain whereas high level signals will be hard- clipped at the predefined maximum level.
• hard- clipping may be implemented by using digital values +1 and -1 as maximum and minimum values, respectively.
• plus and minus signal full scale e.g. plus and minus supply voltage, may be used as the clipping values.
• the method is advantageous for processing audio signals with the purpose of increasing low frequency output of a small loudspeaker, also below its natural cutoff frequency.
• overdriving it is possible to boost the low frequency part of the signal considerably to obtain a larger low frequency output, while too high low frequency signal amplitudes that may damage the loudspeaker or cause distortion are eliminated or at least the risk is highly reduced.
• no distortion components in the form of high frequency tones are added to the signal in spite of the clipping due to the subsequent low-pass filtering in step b).
• overdriving including hard-clipping to obtain a high sound quality.
• the method provides a high signal quality for all low frequency signal types and at all levels, and the clipping enables control of the maximum level applied to a loudspeaker reproducing the output signal, thereby protecting the loudspeaker even though the low frequency range is boosted significantly.
• the overdriving implements a linear gain. Thus it acts as a simple linear bass boost, which does not introduce any undesirable artifacts.
• the method including the hard-clipping overdriving, can be implemented by few and simple processing steps.
• the hard-clipping overdriving is a very simple implementation of the overdriving, and still the clipping serves to protect a connected electro-acoustic transducer by limiting the possible levels of the output signal.
• the processing is a simple linear bass boost with the boost determined by the first gain.
• the first gain may be chosen to be in the range +3 dB to +30 dB, such as in the range +6 dB to +20 dB, such as in the range +8 dB to +16 dB, thereby enabling a significant effective bass boost is obtain, at least at low signal levels. At high signal levels the effective bass boost is reduced due to the overdriving effect, thereby protecting the following electro- acoustic transducer.
• a second low-pass filtering of the input signal is included so as to provide the low frequency signal part.
• a second cut-off frequency of the second low-pass filtering may be substantially equal to the first cut-off frequency of the first low-pass filtering, such as within 1/3-octave, such as within 1/12-octave.
• the first cut-off frequency is selected such that it is within one octave around a low frequency cut-off frequency of an associated electro-acoustic transducer, e.g. a loudspeaker, intended to convert the output signal to an acoustic signal.
• the first cut-off frequency may be substantially equal to the low frequency cut-off frequency of the associated electro-acoustic transducer. With such a choice of the first cut-off frequency, it is ensured, that the processing has its effect below the low frequency cut-off of the transducer.
• the first low- pass filtering has a cut-off steepness of at least 12 dB per octave, such as at least 18 dB per octave, such as 24 dB per octave or even more.
• At least a portion of the input signal is combined with the low frequency signal part prior to performing the overdriving.
• the portion of the input signal includes a high frequency portion of the input signal.
• possible high frequency peaks are taken into account in the overdriving process. This effectively leads to a lower boost of the low frequency signal, thus serving to reduce the risk of clipping distortion in the output signal.
• a second gain may be applied to the input signal before being combined with the low frequency signal part.
• the second gain main be in the range -20 dB to 0 dB, such as -10 dB to -3 dB.
• a high-pass filtering of the input signal to form a high frequency signal part is preferred prior to combining with the low frequency signal part.
• This high frequency signal part can then be combined, in step c), with the processed low frequency signal to form the output signal.
• the high-pass filtering may be chosen to have a cut-off frequency substantially equal to the first cut-off frequency, such as within 1/3-octave, such as within 1/12-octave.
• a further high-pass filtering may be included prior to performing the overdriving, such as high-pass filtering the input signal, or such as high-pass filtering the low frequency signal part.
• a cut-off frequency of this further high-pass filtering is preferably lower than the first cut-off frequency, such as one octave lower than the first cut-off frequency, or such as 2 octaves lower than the first cut-off frequency.
• this further high-pass filter can be used to protect a very small electro-acoustic transducer from high level frequency content significantly below its natural cut-off frequency.
• the method may include applying a third gain of less than zero dB after step b).
• This third gain may be introduced just after the overdriving, before the first low-pass filtering.
• the third gain can be chosen to be in the range -10 dB to - 1 dB, such as in the range -8 dB to -2 dB, such as in the range -6 dB to -3 dB.
• the method may further include performing a signal clipping, e.g. hard-clipping, after combining the signals in step c).
• the method may include attenuating the input signal prior to performing step a).
• the input signal has a level which is already close to full scale, the overdriving of the low frequency signal part will serve to limit the signal level most of the time, and thereby no significant bass boost is obtained.
• attenuating the input signal such as by 3 dB or 6 dB, a considerable increased bass boost can be obtained.
• the method is implemented using simple signal processing steps such as: a simple gain, signal addition, low-pass filtering, and hard-clipping. Complicated limiter or compressor processing can be avoided, and thus the method works sample-by-sample.
• the method is suited to implement either on a digital signal processor with limited processing capacity, or the method may be implemented in an analog electronic circuit, or in a combination of analog and digital processing means.
• the method may be implemented in computer executable program code.
• This code can be present in any computer-readable medium, such as any type of memory, hard disc, portable disc, or memory card etc.
• the invention provides a signal processor arranged to perform the method according to the first aspect.
• the invention provides a device including a signal processor according to the second aspect.
• the device may be such as an audio device, a communication device, a car audio device, a home audio device, a headphone, a personal computer, a TV set, a personal media player (PMP), a gaming console, a hearing aid, a hi-fi device, and accessories to any of the mentioned devices.
• PMP personal media player
• the invention provides a system including a device according to the third aspect, and a loudspeaker arranged to receive the output signal and convert it to a corresponding acoustic signal.
• Fig. 1 illustrates a signal block diagram of basic parts of a preferred embodiment
• Fig. 2a illustrates in schematic form a frequency response of a typical loudspeaker with a low frequency cut-off
• Fig. 2b illustrates preferred bass boost according to the invention in order to provide an optimal result with a typical loudspeaker
• Fig. 3 illustrates an implementation example
• Fig. 4 illustrates another implementation example
• Fig. 5 illustrates a two-channel system according to the invention.
• Fig. 1 illustrates a signal processor SP receiving an input signal X.
• the input signal X is split into a low frequency signal part LSI and a high frequency signal part HSl, where the low frequency signal part LSI is further processed, while the high frequency signal part HSl is not further processed.
• the splitting is performed by means of a low-pass filter LPF2 and a high- pass filter HPF.
• the splitting into LSI and HSl may be obtained in alternative implementations, such as will be described in connection with Figs. 3 and 4. Further, it is to be understood that the splitting is optional, and thus the high-pass filter HPF can be completely left out, thus leaving HSl identical with the input signal X.
• the hard-clipping CLP serves to ensure that the bass boost provided by Gl does not result in an output signal Y leading to too high excursions of the loudspeaker diaphragm causing distortion and possible damage of the loudspeaker.
• the hard- clipping CLP itself, however, introduces distortion.
• the boosted low frequency signal LS3 is filtered by low-pass filter LPFl.
• the function of this low-pass filter LPFl is to eliminate or at least significantly reduce high frequency distortion components introduced by the non-linear hard-clipping CLP process.
• the cut-off frequency of this low-pass filter LPFl is selected approximately equal to the cut-off frequency of low-pass filter LPF2, or possibly slightly lower or slightly higher.
• low-pass filter LPFl is less than one octave higher than the low frequency cut-off frequency of the loudspeaker, preferably less than 1/3 octave higher than the loudspeaker cut-off frequency.
• Low-pass filter LPFl may be implemented digitally as two, preferably three or four cascaded Infinite Impulse Response (HR) low-pass filter sections in order to provide a steep cut-off towards high frequencies, thereby ensuring that high frequency distortion components are effectively attenuated.
• HR Infinite Impulse Response
• Figs. 2a and 2b serve to illustrate preferred interrelations between the properties of an electro-acoustic transducer, e.g. a loudspeaker, intended to convert the output signal Y to a corresponding acoustic signal and the signal processing SP.
• Fig. 2a illustrates a frequency response of a typical loudspeaker with a low frequency cut-off Fl. According to prior art teaching, a perceived bass boost is normally obtained by introducing harmonic tones above Fl, while leaving the frequency range below Fl unprocessed in order to protect the loudspeaker from high levels below Fl.
• the processing according to preferred embodiments of the invention provide a gain below Fl, such as illustrated in Fig. 2b, while the range above Fl is preferably unprocessed by the bass enhancement.
• a linear gain of 10 dB to 15 dB below Fl is provided. This gives an improved bass output at moderate sound levels, while the hard-clipping CLP serves to limit the bass output at high signal levels.
• the cut-off frequencies for two low-pass filters LPFl and LPF2 may both be selected to be equal to, or substantially equal to, the loudspeaker cut-off frequency Fl.
• the cut-off frequency for high- pass filter HPF may also be selected to be equal to, or substantially equal to, the loudspeaker cut-off frequency Fl.
• Fig. 3 illustrates a block diagram which in more details describes a preferred embodiment suited for implementation on a digital signal processor with limited capacity, since only gains, signal additions, low-pass filters and simple hard- clipping are involved.
• reference signs X, Y, Gl, G2, CLP, LPFl and LPF2 see description related to Fig. 1 above, since in the embodiment of Fig. 3 these parts have essentially the same function as described for Fig. 1.
• the embodiment of Fig. 3 is based on an overdriving implemented by means of gain Gl and hard-clipping CLP.
• a gain G3 is included to further scale down the signal level before low-pass filtering in LPFl. This is done to further reduce the risk of signal overload in the low-pass filtering LPFl process.
• G3 may be selected in the range -10 dB to -1 dB.
• FIG. 4 illustrates another implementation example in which one addition point is saved compared to the implementation shown in Fig. 3.
• Functions of reference signs Gl, G3, CLP, LPFl, LPF2, CLP2, G4 and G5 are as described for Fig. 3.
• LPFl however is here sketched as a cascade of four first order IIR low-pass filters.
• G6 and G7 are introduced instead of G2 in Fig.3, i.e. to control the mix of the low frequency signal portion and in this case a linear portion of the input signal which are added prior to the overdriving process.
• the invention provides a method and a device for enhancing low frequency content of an input signal X, e.g. bass boosting of an audio signal.
• An overdriving ODR of a low frequency signal part LSI of the input signal X is performed to produce a boosted low frequency signal LS3, wherein the overdriving (ODR) includes amplifying the low frequency signal part (LSI) by a first gain (Gl) to form an amplified low frequency signal (LS2), and hard-clipping (CLP) the amplified low frequency signal (LS2) to form the boosted low frequency signal (LS3).
• a first low-pass filtering LPFl is then performed, resulting in a processed low frequency signal LS4.
• the first and second cut-off frequencies are preferably selected equal to, or within one octave from, a low frequency cut-off frequency for a loudspeaker intended to reproduce the output signal Y.
• the preferred method introduces a level dependent bass boost below the loudspeaker's low frequency cut-off frequency.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Circuit For Audible Band Transducer (AREA)
• Tone Control, Compression And Expansion, Limiting Amplitude (AREA)

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• 2008
  • 2008-08-20 US US12/676,086 patent/US8582784B2/en active Active
  • 2008-08-20 WO PCT/DK2008/050208 patent/WO2009030235A1/en active Application Filing
  • 2008-08-20 AT AT08784466T patent/ATE520260T1/de not_active IP Right Cessation
  • 2008-08-20 EP EP08784466A patent/EP2191660B1/de not_active Revoked

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